Interpretive Summary: We demonstrated that soils that were cultivated as a means of vineyard weed control had greater soil nitrogen retention than soils that received herbicide. In this specific case, cultivated soils had lower nitrate leaching and greenhouse gas emissions. We attribute this to relative increases in soil organic matter, microbial biomass, and nutrient uptake by weeds in the cultivated treatment as compared to the herbicide treatment. These findings are in contrast to other studies in annual cropping systems that show increases in soil organic matter under no-till conditions. These herbicide and cultivation practices likely influence grapevine N supply and weed pressure, and as such, evaluation of ‘in-row’ weed management practices should include impacts not only on soil N retention but weed establishment, vine N status, and fossil fuel use from additional cultivation, and dust generated by tillage practices that may influence air quality.

Technical Abstract:
Successive years of multiple tillage passes have been linked to reductions in soil carbon (C) and shifts in soil physical properties. ‘In-row’ cultivation is becoming a more common practice in vineyards as growers seek alternative means of weed control. Therefore, we investigated effects of cultivation or herbicide on soil N leaching during a one year cycle of vineyard management and 2) determined differences in nitrous oxide (N2O) emissions and N transformations between ‘in-row’ treatments during summer fertigation, a period in which we hypothesized that high N2O emission rates under warm, moist soil conditions. Few differences were found in the chemical and physical characteristics of soil profiles in cultivated and herbicide treatments. In the surface depths, total C and microbial biomass (i.e., total phospholipid fatty acids) were greater in the tilled than herbicide soils. This contrasts with other studies, but suggests that the greater weed biomass in the tilled treatment contributed labile C to the soil C pools. Greater soil nitrate was found at depth (1.2 m) in the herbicide treatment. Also, the cultivated soil had 50% less N2O emissions during fertigation than the herbicide soil. We suggest that greater weed presence, soil C and microbial biomass in the tilled treatment contributed to the greater N retention. These functional differences in soil N retention were associated with different compositions of soil microbial communities, as measured by PLFA, in the respective weed treatments, suggesting that shifts N retention may have been linked to changes in soil microbial communities.